State of water in various environments: Aliphatic ketones. MIR/NIR spectroscopic, dielectric and theoretical studies

This work provides new insight into the state of water in a series of aliphatic ketones. For our studies, we selected nine aliphatic ketones of different size and structure to examine the effect of various structural motifs on behavior of water in the mixtures. Our results reveal that conformational flexibility of aliphatic chains in the linear ketones allows for effective shielding of the carbonyl group, and this flexibility is the main reason for poor solubility of water. Hence, in the linear ketones molecules of water are involved mostly in ketone-water interactions, while the water-water interactions are rare. Higher solubility of water in the cyclic ketones allows for creation of clusters of water, where the molecules are in water-like environment. The temperature rise in wet cyclic ketones increases population of ketone-water interactions at the expense of the water-water ones, while in the linear ketones and 2,6-dimethylcyclohexanone at an elevated temperature there is an increase in the population of singly bonded water at the expense of the doubly bonded one. DFT calculations reveal that the substitution of cyclohexanone by a single methyl group does not affect the strength of the ketone-water interactions, while it has a significant impact on the solubility of water in the ketone. The most important conclusion from this study is that the accessibility of the carbonyl group is the most important factor determining the intermolecular interactions and solubility of water in aliphatic ketones.

Association and solubility of chlorophenols in CCl4: MIR/NIR spectroscopic and DFT study

This work provides new information on the effect of position and number of substituents on association and solubility of chlorophenols in CCl₄. Using MIR and NIR spectroscopy we examined solutions of 12 chlorophenols at several concentrations. In addition, we calculated (DFT) theoretical spectra and structures of monomers and associates of chlorophenols from dimer to tetramer. The number of substituents at positions 2 and 6 allows to divide studied chlorophenols into three Groups: I (3; 4; 3,4; 3,5), II (2; 2,3; 2,4; 2,5; 2,4,5), and III (2,6; 2,4,6; 2,3,4,5,6). An equilibrium between intermolecular OH⋅⋅⋅OH and intramolecular OH⋅⋅⋅Cl hydrogen bonding depends on position and number of substituents. The extent of association decreases in going from Group I to Group III due to growing steric hindrance near the OH group and the resonance effect from Cl. In chlorophenols of Group I, Cl at positions 3 or 5 weakens the OH⋅⋅⋅OH intermolecular hydrogen bonding, while for Group II it strengthens the OH⋅⋅⋅⋅Cl intramolecular bonding. In contrast, Cl at position 4 has minor effect on association. In the case of Group I, increasing concentration shifts the equilibrium towards solute-solute interactions, whereas for Groups II and III dominate the species with intramolecular OH⋅⋅⋅Cl bonding. The theoretical calculations predict that for monosubstituted chlorophenols of Group I the most stable are non-planar cyclic tetramers, while for disubstituted ones, the non-planar cyclic tetramers and linear trimers have similar binding energies. Chlorophenols of Group II prefer the cyclic non-planar trimers, whereas those of Group III form the planar dimers with an antiparallel orientation of the OH groups. Our study reveals that chlorophenols creating the cyclic associates are better soluble in CCl₄ as compared with those forming the linear ones. Hence, one can conclude that in an inert or weakly interacting solvents the solubility is closely related to the structure of the solute associates.

Solvent Effect on Assembling and Interactions in Solutions of Phenol: Infrared Spectroscopic and Density Functional Theory Study

This work provides new insight into assembling of phenol in various solvents and competition between different kinds of interactions. To examine both weak and strong interactions, we selected a series of non-aromatic and aromatic solvents. Infrared spectra were measured at low (0.05 M) and high (2 M) phenol content. In addition, we performed density functional theory calculations of the structures and harmonic vibrational spectra of 1:1 complexes of phenol with the solvents and the associates of phenol from dimer to tetramer. Based on these results, we divided the solvents into three groups. The first group consists of non-aromatic solvents weakly interacting with phenol. Depending on the concentration, molecules of phenol in these solvents remain non-bonded or self-associated. In diluted solutions of phenol in chlorinated non-aromatic solvents do not appear free OH groups, since they are involved in a weak OH···Cl interaction. It is of note that in diluted solutions of phenol in tetramethyl ethylene both the non-bonded and bonded OH coexists due to solvent-solvent interactions. The second group consists of aromatic solvents with methyl or chlorine substituents. At low concentration, the molecules of phenol are involved in the phenol-solvent OH···π interaction and the strength of these interactions depends on the solvent properties. At a higher phenol content an equilibrium exists between phenol-solvent OH···π and phenol-phenol OH···OH interactions. Finally, the third group includes the aromatic and non-aromatic solvents with highly polar group (C≡N). In these solvents, regardless of the concentration all molecules of phenol are involved in the solute-solvent OH···NC interaction. Comparison of the experimental and theoretical band parameters reveals that molecules of phenol in non-aromatic solvents prefer the cyclic associates, while in the aromatic solvents they tend to form the linear associates.

Solvent effect on the competition between weak and strong interactions in phenol solutions studied by near-infrared spectroscopy and DFT calculations

Near-infrared (NIR) spectra of phenol in a series of non-aromatic and aromatic solvents were recorded to study the competition between various types of solute-solute and solute-solvent interactions. Depending on the phenol concentration, the free OH and OH involved in the OH⋯OH interactions in the dimers and higher associates are present in cyclohexane solutions. On the other hand, free OH does not appear in Cl-containing solvents since at a low phenol content the OH groups participate in the OH⋯Cl interactions. In CCl₄ and tetrachloroethylene this interaction is weak, while in chlorobenzene the strength of this interaction is higher. In the aromatic solvents the solute-solute OH⋯OH interactions compete with the solute-solvent OH⋯π and aromatic CH⋯OH ones. Consequently, the degree of self-association of phenol in aromatic solvents is smaller than that in non-aromatic ones. The strength of the OH⋯π interactions increases with growing electron-donating ability of the substituents in the benzene derivatives. This observation obtained from the NIR spectra is in line with the results of the theoretical calculations (DFT). A clear correlation appears between the number of methyl groups in aromatic solvents and the population of the free OH groups. The methyl groups are steric hindrances and impede the formation of the OH⋯OH and OH⋯π interactions. Our results suggest the presence of aromatic CH⋯OH solute-solvent interactions, not observed in previous studies. NIR spectroscopy appears to be a powerful tool for exploration of free and weakly-bonded OH groups.

How much anharmonicity is in vibrational spectra of CH3I and CD3I?

This work presents new experimental and theoretical insights on vibrational spectra of CH₃I and CD₃I in the liquid phase. For the first time, we provided the contributions from different vibrational modes to mid-infrared (MIR) and near-infrared (NIR) spectra and estimated the extent of anharmonicity in the MIR region. Direct comparison of the intensities from ATR-IR and NIR transmission spectra was possible due to normalization of ATR-IR spectra. As a reference for normalization, we applied the area of the ν_s(CH₃)/ν_s(CD₃) band recorded in transmission mode. Our results show that the corresponding vibrational modes of CH₃I and CD₃I have similar contributions to the total intensity (MIR + NIR), however, these contributions are distributed in a different way between MIR and NIR regions. As expected, most of intensity in MIR spectra originates from the fundamental transitions (>90%). The fundamental bands together with the first overtones and the binary combinations contribute to more than 99% of MIR intensity for both compounds. Therefore, reliable reconstruction of MIR spectra can be achieved by considering only these vibrational modes. On the other hand, accurate simulation of NIR spectra requires including the higher-order transitions. In the case of CD₃I, the fourth-order transitions contribute to 12.7% of NIR intensity. The contributions from NIR region are significantly smaller than those from MIR range and were estimated to be 6.7% for CH₃I and 2.3% for CD₃I. The theoretical calculations provide a reasonable estimation of the total contribution from the fundamental bands. Yet, the calculated contributions from the anharmonic transitions are different from those obtained from the experimental data. MIR spectra of CH₃I and CD₃I reveal an unexpected increase in the intensity of some overtones and combination bands indicating the presence of Fermi resonances. These resonances are responsible for differences in contributions from the first overtones and binary combinations between CH₃I and CD₃I.

Effect of elevated temperature and UV radiation on molecular structure of linoleic acid by ATR-IR and two-dimensional correlation spectroscopy

The effect of elevated temperature (44 °C) and ultraviolet (UV) radiation on molecular structure of linoleic acid (LA) was studied by Attenuated Total Reflection Infrared (ATR-IR) spectroscopy. To obtain more detailed information on molecular mechanism of these changes we applied moving-window analysis and two-dimensional correlation spectroscopy (2DCOS). Analysis of the time-dependent ATR-IR spectra of LA before and after UV irradiation revealed the structural changes in molecules of LA. The extent of these changes was significantly higher after an application of UV radiation. During 24 h experiment temperature was constant, therefore the spectral changes result from relatively slow processes (and requiring more energy), e.g. cis/trans isomerization, disruption of the C=C double bonds and partial breaking of hydrogen bonds in the cyclic dimers. As a side effect of these structural changes one can observe variations in the orientation of the chains. It is of note that the methyl and methylene groups reveal slightly different behaviour.

ATR-IR study of skin components: Lipids, proteins and water. Part II: Near infrared radiation effect

Near infrared (NIR) radiation has been widely used in medicine and biomedical engineering. In spite of numerous studies the molecular mechanism of NIR radiation on biological systems has not been established as yet. The objective of this work was examination of the effect of NIR irradiation on the skin components. Modifications of lipid organization after NIR exposure vs. temperature (from 20 to 90 °C) have been investigated using Attenuated Total Reflectance Infrared (ATR-IR) spectroscopy. This work is a continuation of our previous studies on the temperature effect on skin components [1]. After NIR exposure a temperature shift of the phase transition from the orthorhombic to hexagonal packing (≈40 °C) has been observed. In contrast, the second phase transition temperature (≈70 °C) is almost invariable. The phase transitions in lipids were correlated with modifications of the structure of water and proteins. To our knowledge, for the first time the temperatures of the phase transitions after NIR exposure were investigated.

Microheterogeneity in binary mixtures of water with CH3OH and CD3OH: ATR-IR spectroscopic, chemometric and DFT studies

Here we report ATR-IR spectroscopic study on the separation at a molecular level (microheterogeneity) and the degree of deviation of H₂O/CH₃OH and H₂O/CD₃OH mixtures from the ideal mixture. Of particular interest is the effect of isotopic substitution in methyl group on molecular structure and interactions in both mixtures. To obtain comprehensive information from the multivariate data we applied the excess molar absorptivity spectra together with two-dimensional correlation analysis (2DCOS) and chemometric methods. In addition, the experimental results were compared and discussed with the structures of various model clusters obtained from theoretical (DFT) calculations. Our results evidence the presence of separation at a molecular level and deviation from the ideal mixture for both mixtures. The experimental and theoretical results show that the maximum of these deviations appears at equimolar mixture. Both mixtures consist of three kinds of species: homoclusters of water and methanol and mixed clusters (heteroclusters). The heteroclusters exist in the whole range of mole fractions with the maximum close to the equimolar mixture. At this mixture composition near 55-60% of molecules are involved in heteroclusters. In contrast, the homoclusters of water occur in a limited range of mole fractions (X_ME < 0.85-0.9). Upon mixing the molecules of methanol form weaker hydrogen bonding as compared with the pure alcohol. In contrast, the molecules of water in the mixture are involved in stronger hydrogen bonding than those in bulk water. All these results indicate that both mixtures have similar degree of deviation from the ideal mixture.

ATR-IR study of skin components: Lipids, proteins and water. Part I: Temperature effect

In this work we report the studies of the effect of temperature on skin components, such as lipids, proteins and water. Modifications of lipids structure induced by increasing temperature (from 20 to 90°C) have been studied using ATR-IR (Attenuated Total Reflectance Infrared) spectroscopy, which is a powerful tool for characterization of the molecular structure and properties of tissues, such as skin. Due to the small depth of penetration (0.6-5.6μm), ATR-IR spectroscopy probes only the outermost layer of the skin, i.e. the stratum corneum (SC). The assignment of main spectral features of skin components allows for the determination of phase transitions from the temperature dependencies of band intensities [e.g. ν_as(CH₂) and ν_s(CH₂)]. The phase transitions were determined by using two methods: the first one was based on the first derivative of the Boltzmann function and the second one employed tangent lines of sigmoidal, aforementioned dependencies. The phase transitions in lipids were correlated with modifications of the structure of water and proteins.

Microheterogeneity in binary mixtures of aliphatic alcohols and alkanes: ATR-IR/NIR spectroscopic and chemometric studies

This work evidences microheterogeneity and deviation from the ideality in binary mixtures of aliphatic alcohols and alkanes.

Microheterogeneity in binary mixtures of methanol with aliphatic alcohols: ATR-IR/NIR spectroscopic, chemometrics and DFT studies

This paper reports ATR-IR and NIR spectroscopic studies on microheterogeneity in binary mixtures of methanol with five short chain aliphatic alcohols: ethanol, 1-propanol, 2-propanol,tert-butanol and cyclopentanol.

Molecular structure and hydrogen bonding of 2-aminoethanol, 1-amino-2-propanol, 3-amino-1-propanol, and binary mixtures with water studied by Fourier transform near-infrared spectroscopy and density functional theory calculations

The effect of temperature and water content on the molecular structure and hydrogen bonding of 2-aminoethanol (2AE), 1-amino-2-propanol (2AP), and 3-amino-1-propanol (3AP) has been examined by Fourier transform near-infrared (FT-NIR) spectroscopy. The experimental spectra were analyzed using the two-dimensional (2D) correlation approach and chemometrics methods. Interpretation of the spectra was guided by density functional theory (DFT) calculations. The novelty of the present work relates to the interpretation of the spectra of aminoalcohols in the liquid phase and their mixtures with water based on dimeric structures. The molecules of 2AE and 2AP form stable cyclic dimers through the intermolecular O-H...N hydrogen bonds (HBs), whereas the intramolecular HBs are absent. In contrast, the molecules of 3AP create two kinds of dimers. The first dimer has two intermolecular O-H...N HBs and two intramolecular N-H...O HBs, while the second dimer has the opposite. In the liquid phase the cyclic dimers interact with each other and form higher associates through the intermolecular N-H...O HBs. The temperature rise weakens these interactions but the structure of the dimers remains intact. The majority of the molecules of water act as double proton donors to oxygens linking different molecules of aminoalcohol. This cooperative hydrogen bonding is stronger than that in bulk water. A small amount of one-bonded water occurs in the mixtures, and the population of this species increases with the temperature rise. At higher water content small clusters of water are formed. On the basis of the present results one can conclude that addition of water does not lead to noticeable variations in the structure of liquid aminoalcohols. More significant changes are induced by the temperature variations.

Structural characterization of clear human lens lipid membranes by near-infrared Fourier transform Raman spectroscopy

Regional differences in human lens membrane lipid composition have been documented and could be responsible for alterations in the function of lens membranes. The phospholipid composition of epithelial membranes of human lenses has been shown to be different from that of fiber membranes. To establish lipid composition-membrane structure relationships, we have examined spectroscopically the structure of lipid membranes from human lens epithelium, cortex and nucleus. Near-infrared Fourier transform Raman spectroscopy was used to obtain the lipid structure of membranes in which the lipid composition was determined previously by 31P-NMR. The disorder (fluidity measured structurally) of the epithelium was evaluated to be 80%, whereas that of the lipids from the cortical and nuclear regions was 55%. The large size of the band at 1650 cm-1 arising from sphingolipids supported the compositional studies which indicate that the major component of human lens membranes is a sphingolipid. Sphingolipids probably account for the high degree of lipid order found in lens membranes. Epithelial membranes were found to contain more glycerolipids and less sphingolipids than fiber cell membranes. This compositional difference would be expected to disorder the epithelial membrane.